Cast iron is, in most respects, the easiest common material to turn. The graphite in it acts as a lubricant, chips break into small pieces without any help, and the cutting forces are low. The problems are narrow and specific: abrasive wear, the casting skin, and interruption.
Real cutting data for ISO K
Aggregated from 1160 insert records published for cast iron in our catalogue:
| Parameter | Full published range | Typical (median) |
|---|---|---|
| Cutting speed vc | 50 – 490 m/min | 95 – 400 m/min |
| Feed fn | 0.02 – 1.50 mm/rev | 0.10 – 0.40 mm/rev |
| Depth of cut ap | 0.10 – 10.00 mm | 0.7 – 3.5 mm |
Grey cast iron (GJL/GG) sits at the top of that range. Ductile / nodular iron (GJS/GGG) is tougher and needs about 20–30% less speed. Austempered ductile iron (ADI) is harder again.
Grades that work
Our most-stocked cast iron grades: AT202, AC102K, AC202K, AP251K, AC301K, AC100K, HS6120, AC151K, PB90, AC251P.
Cast iron grades favour a tough substrate with a thick CVD coating, because the dominant wear mode is abrasion rather than heat. A fine-grain grade designed for steel finishing will abrade quickly here.
Handling the casting skin
The as-cast surface carries sand, scale and occasional hard spots. It will destroy an insert edge at normal speeds. Standard practice:
- Reduce vc by 30–50% on the first pass that crosses the skin.
- Increase ap so the cut goes under the skin rather than along it — at least 1–2 mm into clean metal.
- Use a tough grade and a honed edge for that first pass, then switch to a wear-resistant grade for the bulk.
- Where possible, start in a machined area or pre-machine the entry with a grinder.
Dry or wet?
Grey cast iron is normally machined dry. The reasons are practical: the chips are small and easy to extract, coolant turns them into a paste that packs into the machine, and thermal cycling from coolant can crack the edge. Dry is also cheaper and cleaner.
Use coolant when: machining ductile iron at high speed, holding tight tolerances that need thermal stability, or when the dust has to be suppressed for health reasons. If you do use coolant on grey iron, use it continuously, never intermittently — intermittent coolant is what causes thermal cracking.
Geometry and chipbreakers
Because chips break by themselves, the chipbreaker matters less in cast iron than in steel. What matters more is edge strength:
- Use the largest nose radius the profile allows — it resists abrasion and spreads heat.
- Prefer negative geometry (CNMG, WNMG, SNMG) for roughing, or round inserts (RNMG) for the hardest skins and heaviest interruption.
- Chipbreakers most used for K in our catalogue: KD5, KC4, MM4, KC2, PB1, MV.
Wear to expect
- Flank wear — normal and gradual. Target failure mode.
- Abrasion / micro-chipping at the edge — speed too high, or grade too soft, or you are cutting sand inclusions.
- Edge breakage on the skin — reduce speed on entry, increase ap, tougher grade.
Where to start
- General grey iron: CNMG 120408 with a K grade, vc 150–300, fn 0.15–0.35, ap 1–4
- Heavy roughing, scale, interruption: SNMG 120408 or round inserts
- Face milling cast iron: milling inserts with K grades, run dry
Related
Standards and further reading
The material group definitions used throughout this site follow ISO 513. The manufacturer links below are independent technical references.